United Airlines just fired up a tool on their website called Award Accelerator, a tool that essentially lets you pay to double or triple earned miles on any of your flights. Basically, you pay a sliding fee on check-in that effectively equates to three cents per mile, then at the end of your flight the airline dumps the bonus into your account.

So say you’re flying 500 miles from Detroit to New York City. Pay fifteen bucks and you can earn 1000 miles for the trip, or pay thirty and you can earn 1500.

So when is this useful?

Well, if you divide the cost of a regular domestic award ticket, 25,000 miles, by 0.03 cents/mile, you get a total of 750$ spent for those miles. In other words, if you bought all of your miles for an award ticket this way you would be paying 750$ for that ticket — too much for any old domestic ticket. So it doesn’t make sense to double or triple your miles on every flight.

The instance in which this tool might be handy, however, is when you need just a few extra miles to reach that award tier from which you can book. You’re at 24,000 miles, you’ll earn 500 on your next trip and want to push that to 25,000, for example. In this case, it makes sense.